Birds’ Tiny “Microchromosomes” Have Barely Changed In 684 Million Years

Where mammals' chromosomes are all of similar sizes, birds and most reptiles have a mix of macro- and microchromosomes, the latter of which have remained remarkably conserved. The chromosomes shown here, with most microchromosomes huddled together as frequently happens, are of the central bearded dragon, known for their strange sex-determination system. Image Credit: Shayer Alam

Birds and reptiles hold some of their genes in microchromosomes so small they were once thought to be dust on the microscope slide. A comparative study across many species has found most birds have been preserving similar microchromosomes since before their ancestors had spines, let alone wings. Meanwhile, the same bits of DNA have joined ordinary chromosomes in mammals, except the platypus, which is doing its own thing, as usual.

Human chromosomes contain approximately 3 billion base pairs of DNA, with lengths of around 3 to 6 µm (0.00012-0.00024 inches). Birds and reptiles have a smaller number of similar-sized macrochromosomes, along with 30-32 microchromosomes about a tenth as long.

Proceedings of the National Academy of Sciences paper reveals the microchromosomes in a chicken are similar to those in an Amphioxus, a sort of spineless deep-sea creature that diverged from the vertebrate line 684 million years ago. Most other birds have kept the same set of microchromosomes as well, the paper reveals, with a few odd exceptions like eagles and some parrots.

“We lined up these sequences from birds, turtles, snakes and lizards, platypus and humans and compared them,” Professor Jenny Graves of La Trobe University said in a statement. “Astonishingly, the microchromosomes were the same across all bird and reptile species. Even more astonishingly, they were the same as the tiny chromosomes of Amphioxus.”

Graves and co-authors were able to identify legacies of a few microchromosomes in patches of human chromosomes, but most mammals have absorbed and scrambled the microchromosomes so thoroughly they seem like any other bits of DNA. “The exception is the platypus genome, in which the microchomosomes have all fused together into a few large blocks,” Graves said.

Graves told IFLScience the platypus almost certainly served as a stepping stone to the mammalian approach to chromosomes, a stage the earliest mammals went through, but only monotremes have maintained

Microchromosomes are rich in genes for their size, the paper notes, containing less non-coding “junk DNA” than macrochromosomes. There also seems to be no pattern as to the function of microchromosome genes. However, knowing this beforehand didn't make Graves any less surprised to discover how well some species have preserved them at the same time as genes have shuffled between chromosomes in others.

Graves told IFLScience geneticists have spent 50 years debating whether there is an evolutionary benefit to fusing DNA into large chromosomes or splitting it up. “For genes that work together well it may help to have them on the same chromosome,” Graves explained, but in most cases, the fusion or fission may reflect the invasion of transposable elements, often from old viruses, with no evolutionary benefit or harm.

However, Graves admitted to IFLScience she does not understand how cells move a mix of large and small chromosomes around when bird cells need to replicate. “I don't think we have thought hard enough about the mechanics of getting both into the daughter cell.”

Another mystery is how the shift relates to the formation of new species. What happens when an animal with lots of microchromosomes mates with one with only the larger version? “That's a really deep question,” Graves told IFLScience, and geneticists have been puzzling over similar topics since the 1930s. “I suspect when you have a major chromosome change in a population offspring don't do so well, you have gametes with too many of some genes and too few of others.” How this doesn't impede the process of forming new species, no one knows.

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